As the requirements for controlling pollution from smoke stacks become more stringent, it is important that suitable monitoring equipment be provided to measure the extent and composition of particulate matter and gases from these stacks. These stack gases are often "dirty" and "wet", i.e., they are laden with particulate material and have very high moisture content, all of which tends to damage or clog the measuring equipment. This often results in breakdown of the equipment and/or in faulty readings. The cost of maintenance and repair and down time can be excessive.
One prior art gas sampling device is an extractive probe manufactured and sold by Lear Siegler Measurement Controls Corporation of Englewood, Colo., Model No. 80280366. The extractive probe is mounted on the sidewall of a stack. It has a probe which extends in toward the center of the stack and an external filter outwardly of the stack and in fluid communication with the probe. A flange is provided which attaches the device to the sidewall. The filter is in fluid communication with a vacuum pump which draws gas from the stack through the probe and the filter so that it may be supplied to an analyzer which measures the level of a particular gas under surveillance, such as SO.sub.2. The filter separates out any particulate material that may be in the gas stream in the stack so that it does not pass through the pump or into the analyzer. A typical filter is porous having many small openings. As a result, the gas increases in velocity as it passes through these openings in the filter. As this happens, the gas pressure drops and if the gas is wet, the moisture may exceed the dew point and condense out into the filter, thereby clogging it. To minimize this possibility, the filter is heated. Also, the lines through which the sample gas passes are also heated. While this equipment generally works well for its intended purpose, the heating of the sample lines is expensive and increases the maintenance requirements.
Another prior art device is a dilution probe extractive system sold by Lear Siegler Measurement Controls Corporation of Englewood, Colo. under the trademark "Dynatron 5000", which has particular application in measuring SO.sub.2, NO, NO.sub.2, NO.sub.x, CO, CO.sub.2 and HCL. This device includes a probe which is mounted on the sidewall of a stack as described above. The sample gas is drawn through a filter to remove particulates from the sample before the gas reaches a critical orifice provided within the probe. The flow of the gas through the critical orifice is at sonic velocities. The gas then passes through a primary nozzle where it is mixed with a dilution gas. The diluted gas passes through an eductor at sonic velocity and is supplied to an analyzer. This technique allows a fairly constant flow rate of gas through the probe over a wide range of external process temperatures and pressure variations. Since no water is removed from the diluted sample process stream, measurements are on a volumetric, wet basis. However, when the gas moves at sonic velocities the pressure drops and if the gas is wet the moisture may condense out and clog the critical orifice thereby impeding the operation and efficiency of the dilution probe.